Optical fiber having an elevated threshold for stimulated brillouin scattering

ABSTRACT

An optical fiber having an elevated threshold for stimulated Brillouin scattering is provided. The optical fiber includes a core and a cladding surrounding the core with both the core and the cladding designed to guide optical waves through the core while anti-guiding acoustic waves. Moreover, the optical fiber includes other features to alter the mode profile of the acoustic waves and/or to further promote their lateral radiation. For example, the optical fiber can include an irregular coating to alter the mode profile of the acoustic waves. In another example, the optical fiber can include a quarter wave layer surrounding the cladding to promote the lateral radiation of the acoustic waves. In order to further alter the mode profile of the acoustic waves, the cladding can also have a lateral thickness that varies irregularly in a lengthwise direction.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of U.S. application Ser. No.09/795,156, filed Feb. 28, 2001, which is hereby incorporated herein inits entirety by reference.

FIELD OF THE INVENTION

[0002] The present invention relates generally to optical fibers and,more particularly, to optical fibers having an increased threshold forstimulated Brillouin scattering.

BACKGROUND OF THE INVENTION

[0003] In many applications, such as high power fiber optic industriallasers and scaleable high power fiber optic phased array laser systems,it is desirable to transmit optical signals having substantial amountsof power via optical fibers. Unfortunately, stimulated Brillouinscattering oftentimes limits the amount of power that can be transmittedvia an optical fiber such that, even as additional input power isprovided, the output power remains relatively fixed at the threshold atwhich stimulated Brillouin scattering commences.

[0004] In general, stimulated Brillouin scattering is a phase-matchedparametric amplification process involving the coupling of a opticalwave, an acoustic wave and a backward propagating Stokes wave. In thisregard, variations in the index of refraction of an optical fiberinduced by pressure differences created by an acoustic wave travelingalong the optical fiber can cause a portion of the optical wave to bebackscattered, thereby creating the backward propagating Stokes wave.The backward propagating Stokes wave essentially robs power from theoptical wave so as to limit the power of the optical signals that can betransmitted via the optical fiber. With reference to quantum physics,stimulated Brillouin scattering can therefore be described by thetransfer of a photon from the optical wave into a new Stokes photon oflower frequency and the creation of a new phonon that adds to theacoustic wave.

[0005] With reference to FIG. 1, as the input power of the signaltransmitted via an optical fiber is increased up to the threshold forstimulated Brillouin scattering, the power level of the signals outputby the optical fiber similarly increases as evidenced by positive slopeof curve 10. Upon reaching the threshold for stimulated Brillouinscattering, however, further increases in the power of the signalstransmitted via the optical fiber will not translate into increasedpower levels of the optical signals output by the optical fiber.Instead, the power level of the optical signals output via the opticalfiber will remain at the threshold at which stimulated Brillouinscattering commences as evidenced by the horizontal portion of curve 10,while the additional input power will be transferred to the backwardpropagating Stokes wave as shown by the positive slope of curve 12.

[0006] Parametric processes, such as stimulated Brillouin scattering,are enhanced in guided wave structures in general, and optical fibers inparticular, because the waves that interact, i.e., the optical waves,the acoustic waves, and the Stokes waves, are maintained in the coreover relatively long distances. Moreover, stimulated Brillouinscattering is particularly apparent in optical fibers that exhibit asignificant overlap of the fundamental optical and acoustic modes withinthe core of the optical fiber. In this regard, an overlap integral isdefined as the integral of the product of the acoustic wave amplitudeand the optical wave amplitude over the lateral cross-sectional area ofthe optical fiber. As the overlap integral approaches unity, couplingbetween the optical waves and the acoustic waves is at a maximum,thereby resulting in a high level of stimulated Brillouin scattering. Asdepicted in FIG. 2, for example, a conventional optical fiber having acore doped with GeO₂ is susceptible to the early onset of stimulatedBrillouin scattering since the fundamental optical and acoustic modeshave a 67% mode overlap in the core for an optical wavelength of 1.55microns and an acoustic frequency of 11.25 GHz. Thus, the forwardpropagating optical wave of such an optical fiber will couple energyinto the formation of a longitudinal acoustic wave which, in turn, canreflect a portion of the power carried by the optical wave back towardthe source.

[0007] In order to avoid the limitations imposed by the threshold atwhich stimulated Brillouin scattering commences, optical systems aretypically designed such that the optical fibers are operated below thethreshold for the onset of stimulated Brillouin scattering. As will beapparent, this approach effectively limits the performance andscalability of the optical systems and may effectively prevent theoptical system from being utilized for applications demanding highenergy levels. Alternatively, some optical systems utilize a pluralityof optical fibers such that the total power handling capability of theplurality of optical fibers satisfies the power requirements of theparticular application while ensuring that the power of the opticalsignals transmitted via each optical fiber is below the threshold atwhich stimulated Brillouin scattering commences. While facilitating thedelivery of optical signals having increased power levels, opticalsystems of this type obviously include an increased number ofcomponents, thereby leading to increased costs and increased weight andvolume requirements. Thus, it would be desirable to provide an improvedtechnique for optically transmitting relatively large amounts of power,such as power levels that exceed the threshold at which stimulatedBrillouin scattering would commence in a typical optical fiber, suchthat lasers and other high energy optical systems can be developedwithout requiring the use of multiple optical fibers that unnecessarilyincrease the weight and volume of the optical system.

SUMMARY OF THE INVENTION

[0008] An optical fiber having an elevated threshold for stimulatedBrillouin scattering is therefore provided. The optical fiber includes alongitudinally extending core and a cladding surrounding the core andextending lengthwise therealong, wherein both the core and the claddingare specifically designed to guide optical waves through the core whileanti-guiding acoustic waves. Moreover, the optical fiber includes otherfeatures to alter the mode profile of the acoustic waves and/or tofurther promote the lateral radiation of at least some of the acousticwaves. The threshold for stimulated Brillouin scattering can thereforebe increased relative to a conventional optical fiber since the forwardpropagating optical wave cannot couple energy into the formation of alongitudinal acoustic wave as readily as in conventional optical fibersdue to the anti-guiding of the acoustic waves and the alterations of themode profile.

[0009] The core of the optical fiber of the present invention has afirst index of refraction and a first acoustic wave propagationvelocity. Similarly, the cladding has a second index of refraction thatis less than the first index of refraction of the core and a secondacoustic wave propagation velocity that is less than the first acousticwave propagation velocity of the core. In order for the core and thecladding to have indices of refraction and acoustic wave velocities withthe proper relationship, the optical fiber of one embodiment has a corethat includes aluminum oxide as a dopant and/or a cladding that includesfluorine or boron oxide as a dopant. As a result of the relationship ofthe indices of refraction and acoustic wave velocities, optical wavescan be guided through the core, while the acoustic waves are radiatedaway from the core and into the cladding, i.e., the acoustic waves areanti-guided. Due to the guiding of the optical waves and theanti-guiding of the acoustic waves, the fundamental optical and acousticmodes will not overlap as much within the core as in conventionaloptical fibers and the threshold for stimulated Brillouin scatteringwill be accordingly increased.

[0010] In one embodiment, the optical fiber further includes anirregular coating disposed on the cladding that varies in a lengthwisedirection in order to alter the mode profile of the acoustic waves. Forexample, the irregular coating can be an acoustically dampening materialthat is acoustically matched to the cladding. As such, acoustic wavesthat reach the interface of the cladding and the coating will continueto radiate laterally from the cladding into the coating for furtherdampening. In order to couple the fundamental acoustic mode into higherorder acoustic modes which provide little, if any, power to thestimulated Brillouin scattering process and to incoherently scatteracoustic energy back into the cladding and the core, the coating isirregular. For example, the coating can have a lateral thickness thatvaries randomly in a lengthwise direction. Alternatively, the coatingcan have a density that varies randomly in a lengthwise direction.Further, the coating can include a plurality of segments havingdifferent lengths that are spaced apart in a lengthwise direction. Stillfurther, the coating can include a plurality of segments that are spacedapart in a lengthwise direction by gaps of different lengths. Regardlessof the type of irregularity, the coating is designed to change the modeprofile of the acoustic waves that reach the interface between thecladding and the coating such that any acoustic waves that are scatteredback into the cladding and/or the core will have a negligible influenceon the optical waves guided through the core.

[0011] According to another embodiment, the optical fiber includes acore and a cladding as described above along with a quarter wave layerdisposed on and extending lengthwise along the cladding. In order topromote the lateral or radial radiation of the acoustic waves that reachthe interface between the cladding and the quarter wave layer, thequarter wave layer has a thickness that equals an odd multiple of aquarter of a predetermined Brillouin scattering wavelength. By promotingthe radial or lateral radiation of the acoustic waves away from the coreand cladding of the optical fiber, the threshold at which stimulatedBrillouin scattering commences is further increased.

[0012] In order to further alter the mode profile of the acoustic waves,the cladding can have a lateral thickness that varies irregularly in alengthwise direction. Thus, any acoustic waves reflected from onelocation of the exterior surface of the cladding back toward the corewill be out of phase from acoustic waves that may be reflected from theexterior surface of the cladding at other locations along the length ofthe optical fiber. Accordingly, the irregular lateral thickness of thecladding of the optical fiber of this embodiment further serves toincrease the threshold at which stimulated Brillouin scatteringcommences.

[0013] The optical fiber of the present invention therefore provides anincreased threshold at which stimulated Brillouin scattering commences.As such, the optical fiber of the present invention can be utilized todeliver optical signals having increased power levels relative toconventional optical fibers and is therefore particularly suitable forapplications, such as high power fiber optic industrial lasers andscalable high power fiber optical phased array laser systems, thatrequire the transmission of optical signals having power levels wellabove the threshold for stimulated Brillouin scattering of conventionaloptical fibers.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Having thus described the invention in general terms, referencewill now be made to the accompanying drawings, which are not necessarilydrawn to scale, and wherein:

[0015]FIG. 1 is a graph illustrating the transfer of input power fromthe optical signals propagating along an optical fiber in a forwarddirection to backscattered optical signals propagating in the oppositedirection;

[0016]FIG. 2 is a graph depicting the overlap between the fundamentaloptical and longitudinal acoustic modes of a conventional optical fiberhaving a core doped with GeO₂;

[0017]FIG. 3 is a cross-sectional plan view of an optical fiberaccording to one embodiment of the present invention in which theirregular coating has a lateral thickness that varies randomly in alengthwise direction;

[0018]FIG. 4 is a cross-sectional plan view of an optical fiberaccording to another embodiment of the present invention in which theirregular coating includes a plurality of segments having differentlengths that are spaced apart in a lengthwise direction;

[0019]FIG. 5 is a cross-sectional plan view of an optical fiberaccording to another embodiment of the present invention in which theirregular coating includes a plurality of segments that are spaced apartin a lengthwise direction by gaps of different lengths;

[0020]FIG. 6 is a cross-sectional plan view of an optical fiberaccording to another embodiment of the present invention that includes aquarter wave layer disposed on and extending lengthwise along thecladding; and

[0021]FIG. 7 is a cross-sectional plan view of an optical fiber ofanother embodiment of the present invention in which the cladding has alateral thickness that varies irregularly in a lengthwise direction.

DETAILED DESCRIPTION OF THE INVENTION

[0022] The present invention now will be described more fullyhereinafter with reference to the accompanying drawings, in whichpreferred embodiments of the invention are shown. This invention may,however, be embodied in many different forms and should not be construedas limited to the embodiments set forth herein; rather, theseembodiments are provided so that this disclosure will be thorough andcomplete, and will fully convey the scope of the invention to thoseskilled in the art. Like numbers refer to like elements throughout.

[0023] Referring now to FIG. 3, an optical fiber 20 according to oneembodiment of the present invention is depicted that exhibits anelevated threshold for stimulated Brillouin scattering relative toconventional optical fibers. The optical fiber includes a longitudinallyextending core 22 having a first index of refraction and a firstacoustic wave propagation velocity. The optical fiber also includes acladding 24 surrounding the core and extending lengthwise therealong.Like the core, the cladding includes a second index of refraction and asecond acoustic wave propagation velocity.

[0024] While both the core 22 and the cladding 24 are typically formedof silica, at least one of the core and the cladding is preferably dopedsuch that the first index of refraction of the core is greater than thesecond index of refraction and the cladding such that the optical wavesand, in particular, the fundamental mode of the optical waves areconfined within and guided through the core. According to the presentinvention, at least one of the core and the cladding are also doped suchthat the first acoustic wave propagation velocity of the core is greaterthan the second acoustic wave propagation velocity of the cladding. Inthis regard, the acoustic wave propagation velocity of a material isdetermined by the square root of the ratio of material density to theelastic stiffness constant. By doping at least one of the core and thecladding such that the first acoustic wave propagation velocity of thecore is greater than the second acoustic wave propagation velocity ofthe cladding such that the acoustic waves and, in particular, thefundamental mode of the acoustic waves is not confined within and guidedthrough the core. Instead, the optical fiber 20 of the present inventionanti-guides the acoustic waves such that the acoustic waves arepermitted to radiate laterally or radially out of the core and throughthe cladding toward the exterior surface of the cladding.

[0025] In order for the core 22 and the cladding 24 to have the desiredindices of refraction and the desired acoustic wave propagationvelocities, the core and/or the cladding must be appropriately doped. Inthis regard, the core can include aluminum oxide as a dopant. Inaddition to or instead of doping the core with aluminum oxide, thecladding can be doped with either fluorine or boron oxide.

[0026] By guiding the optical waves while antiguiding the acousticwaves, the optical fiber 20 of the present invention preventssignificant mode overlap between the fundamental optical and acousticmodes within the core 22 which otherwise facilitates the coupling ofenergy from the forward propagating optical wave into a longitudinalacoustic wave which, in turn, can reflect the optical mode back towardsthe source by means of stimulated Bruillouin scattering. In other words,the optical fiber reduces the overlap integral by spreading the acousticmode much broader than the optical mode, thereby delaying the onset ofstimulated Brillouin scattering.

[0027] The optical fiber 20 of the present invention is also designed,however, to prevent or otherwise alter the reflection of the acousticwaves from the exterior surface of the cladding 24 since the unalteredreflection of the acoustic waves by the exterior surface of the claddingwould otherwise cause the acoustic mode profile to only weakly fall offwith radial distance from the longitudinal axis of the optical fiber. Inthis regard, the optical fiber can be designed to prevent or otherwisealter reflections of the acoustic waves from the exterior surface of thecladding without correspondingly altering the optical waves propagatingthrough the core 22 since the fundamental optical mode falls off muchmore quickly in the radial or lateral direction than the fundamentalacoustic mode. For example, the amplitude of the fundamental opticalmode is typically down by over three orders of magnitude at the exteriorsurface of the cladding from its value along the longitudinal axis ofthe optical fiber due to the exponential radial decay of the fundamentaloptical mode in the cladding.

[0028] According to one embodiment, the optical fiber 20 of the presentinvention therefore further includes an irregular coating 26 disposed onthe cladding 24. The irregular coating varies in a lengthwise directionin order to alter the mode profile of the acoustic waves. Preferably,the irregular coating is comprised of an acoustically dampening materialthat is acoustically matched to the cladding. Thus, acoustic waves thatreach the interface between the cladding and the coating will continueto propagate radially or laterally outward into the coating so as to bedampened therein. While a variety of materials are acousticallydampening and can be acoustically matched to the cladding, the irregularcoating is preferably formed of a material that can be readilymechanically coupled to the exterior surface of the cladding, such as byphysical vapor deposition, chemical vapor deposition, epoxy adhesion,metallization and soldering, photolithography or liquid or vapor etchingor that can be extruded along with the core and cladding of the opticalfiber. For example, the irregular coating of one embodiment is formed ofchalk-fast orange resin, while the coating of other embodiments is acomposite material consisting of a plurality of particles embedded in amatrix, such as a plurality of lead particles embedded in a matrix ofpoly(methyl methacrylate) (PMMA) or a plurality of tungsten particlesembedded in an epoxy or in room temperature vulcanization (RTV)silicone, that can be extruded along with the core and the cladding.

[0029] In addition to being formed of an acoustically dampeningmaterial, the coating 26 is irregular so as to vary in a lengthwisedirection along the optical fiber 20 such that in the acoustic wavesthat are reflected by the coating or from the interface between thecladding 24 and the coating toward the core 22 do not substantiallycontribute to the fundamental acoustic mode within the core. In thisregard, the irregular coating can couple the fundamental acoustic modeinto higher order acoustic modes which provide little, if any, power forstimulated Brillouin scattering. In addition, the irregular coating canincoherently scatter acoustic energy back into the cladding and the corein order to actually interfere with the fundamental acoustic mode. Inthis regard, due to the irregularities of the coating, the phase of theacoustic wave reflected at one location along the length of the opticalfiber will generally be different than the phase of the acoustic wavereflected at another location along the length of the optical fiber inorder to cause at least some interference between the various acousticmodes. As such, the reflected acoustic waves may create a fundamentalacoustic mode having a non-planer phase front within the core of theoptical fiber, such as a fundamental acoustic mode having side lobes ofopposite polarity which substantially reduce the overlap integralbetween the fundamental optical and acoustic modes within the core,thereby increasing the threshold at which stimulated Brillouinscattering commences.

[0030] As depicted in FIG. 3, the irregular coating 26 can have alateral thickness that varies randomly in a lengthwise direction inorder to alter the mode profile of the acoustic waves. In addition to orinstead of having a lateral thickness that varies randomly in thelengthwise direction, the irregular coating can have a density thatvaries randomly in the lengthwise direction in order to also alter themode profile of the acoustic waves. As depicted in FIG. 4, the irregularcoating of the optical fiber of another embodiment includes a pluralityof segments that are spaced apart in the lengthwise direction that havethe same lateral thickness, but different lengths. The plurality ofsegments can have respective lengths that are randomly selected or thatare selected in another manner. For example, the plurality of segmentsof the irregular coating of the embodiment of FIG. 4 can have lengthsthat are related to one another by ratios of prime number, such as 3:7,7:19, 1:5 and the like. As depicted in FIG. 5, the optical fiber 20 ofanother embodiment includes an irregular coating that also has aplurality of segments that can be either the same or different lengths,but that are spaced apart in a lengthwise direction by gaps 27 ofdifferent lengths. In this embodiment, the gaps can have lengths thatare randomly selected or that are related to one another in some othermanner. As described above in conjunction with the plurality of segmentsof FIG. 4, the gaps can have respective lengths that are related to eachother by ratios of prime numbers. In either embodiment, however, theplurality of segments that are separated by respective gaps also serveto alter the mode profile of the acoustic waves.

[0031] While the irregular coatings 26 of the embodiments of the opticalfibers 20 depicted in FIGS. 3-5 and described above are effective inorder to alter the mode profile of the acoustic waves in a manner thatincreases the threshold at which stimulated Brillouin scatteringcommences, the optical fiber of the present invention can include otherfeatures in addition to or instead of the irregular coating in order tosimilarly alter the mode profile of the acoustic waves. In this regard,the optical fiber can include a quarter wave layer 28 disposed on andextending lengthwise along the cladding 24, as shown in FIG. 6. Like theirregular coating, the quarter wave layer is preferably formed of amaterial that is acoustically dampening and that is acoustically matchedto the cladding. For example, the quarter wave layer can be formed of aglass or a plastic, such as silicon dioxide. The quarter wave layer hasa thickness that equals an odd multiple at a quarter of a predeterminedBrillouin scattering wavelength. In this regard, the predeterminedBruillouin scattering wavelength is generally one to a few micrometers,such as about 1.1 microns. The quarter wave layer therefore effectivelyserves as an anti-reflectance coating on the cladding in order topromote radial or lateral radiation of the fundamental acoustic modeaway from the core 22. As such, the optical fiber of this embodimentalso serves to reduce the magnitude of the fundamental acoustic modewithin the core and thereby increases the threshold at which stimulatedBrillouin scattering would commence. In some embodiments, the opticalfiber may include both a quarter wave layer and an irregular coating,typically with the irregular coating disposed upon the quarter wavelayer.

[0032]FIG. 7 depicts another embodiment of the optical fiber 20 of thepresent invention which does not include a coating or other structure onthe cladding 24, but which is, instead, designed such that claddingitself has a lateral thickness that varies irregularly in a lengthwisedirection. In this regard, the thickness of the lateral thickness of thecladding can vary randomly or in some other irregular manner along thelength of the optical fiber. Due to the irregular lateral thickness ofthe cladding, the optical fiber of this embodiment also serves to alterthe mode profile of the acoustic waves so as to further increase thethreshold at which simulated Brillouin scattering commences. In thisregards, the phase of the acoustic signals reflected by the exteriorsurface of the cladding at different locations along the length of theoptical fiber will be different such that the reflected acoustic wavesinterfere with one another and contribute little, if any, to thefundamental acoustic mode within the core 22 of the optical fiber. Whilethe optical fiber of FIG. 7 does not include any type of coating overthe cladding, the optical fiber of this embodiment can include both acladding with an irregular lateral thickness as well as a coating, suchas an irregular coating 26 or a quarter wave layer 28 disposed on thecladding.

[0033] Other techniques can also be implemented in conjunction with theoptical fiber 20 of the present invention in order to further couplepower away from the fundamental acoustic mode that otherwise supportsthe stimulated Brillouin scattering process. For example, shear wavescan be externally generated and applied to the optical fiber so as topropagate laterally or radially through the optical fiber. The shearwaves will also couple power away from the fundamental longitudinalacoustic mode and correspondingly increase the threshold at whichstimulated Brillouin scattering will occur.

[0034] The optical fiber 20 of the present invention therefore providesan increased threshold for the commencement of stimulated Brillouinscattering relative to conventional optical fibers. As such, the opticalfiber of the present invention can be utilized to deliver opticalsignals having increased power levels relative to conventional opticalfibers and is therefore particularly suitable for applications, such ashigh power fiber optic industrial lasers and scalable high power fiberoptic phased array laser systems, that require the transmission ofoptical signals having power levels well above the threshold forstimulated Brillouin scattering of conventional optical fibers.

[0035] Many modifications and other embodiments of the invention willcome to mind to one skilled in the art to which this invention pertainshaving the benefit of the teachings presented in the foregoingdescriptions and the associated drawings. Therefore, it is to beunderstood that the invention is not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation.

That which is claimed:
 1. An optical fiber having an elevated thresholdfor stimulated Brillouin scattering comprising: a longitudinallyextending core having a first index of refraction and a first acousticwave propagation velocity; a cladding surrounding said core andextending lengthwise therealong, said cladding having a second index ofrefraction that is less than the first index of refraction of said core,said cladding also having a second acoustic wave propagation velocitythat is less than the first acoustic wave propagation velocity in orderto guide optical waves through said core while antiguiding acousticwaves; and an irregular coating disposed on said cladding that varies ina lengthwise direction in order to alter a mode profile of the acousticwaves, wherein said irregular coating is comprised of an acousticallydampening material that is acoustically matched to said cladding.
 2. Anoptical fiber according to claim 1 wherein said irregular coating iscomprised of an acoustically dampening material that is acousticallymatched to said cladding.
 3. An optical fiber according to claim 1wherein said irregular coating has a density that varies randomly in alengthwise direction.
 4. An optical fiber according to claim 1 whereinsaid core comprises aluminum oxide as a dopant.
 5. An optical fiberaccording to claim 1 wherein said cladding comprises a dopant selectedfrom the group consisting of fluorine and boron oxide.
 6. An opticalfiber according to claim 1 wherein said cladding has a lateral thicknessthat varies irregularly in a lengthwise direction in order to alter amode profile of the acoustic waves.